The old ComBloc actually has a very unique array of calibers. Most of their rew-served weapons can be found on this list:
http://www.fas.org/man/dod-101/sys/land/row/crewwpns_frame.htm
As for why they have two heavy machine guns, that is a good question. Their 12.7mm round is 1cm longer than ours. Their 14.5mm round is huge! The 12.7mm is mostly anti-air and ground use, but the 14.5 is used a lot for anti-vehicle and anti-air use. Both are impressive rounds.